IgE is a heterotetramer composed of two heavy chains and two light chains held together by disulfide bonds forming three regions separated by a protease sensitive section. The two identical Fab regions bind antigens and the single Fc region is responsible for effector functions, including binding to FCC receptors. The overall structure is similar to IgG, except that there is an additional C domain (Cε4) in the ε heavy chain of IgE relative to the γ heavy chain of IgG.
Both IgE and IgG are N-glycosylated. However, IgG has only one N-linked glycan at position Asn-297 of the γ-chain. Human IgE has seven N-linked glycans attached to the heavy ε-chain at different sites. The overall structures of IgG and IgE are shown in FIG. 1. Glycosylation sites are also indicated.
The detailed structure and composition of the glycans on IgE are not known, but the most common structure contains two N-acetylglucosamine (GlcNAc) residues in the base and a high density of mannose residues. Several glycans are located in the Fc-region of IgE; Asn-265 in the Cε2 domain, and Asn-371 and Asn-394 in the Cε3 domain. In addition, IgE from non-myeloma can have a further glycan at Asn-383 in the Cε3 domain
The Asn-297-linked-glycan on IgG is of the complex biantennary type with a core fucose linked to the innermost GlcNAc. The glycan of each γ heavy chain is located in the interface between the Cγ2 domains (second constant domain of the γ heavy chains). Sequence alignment between IgG, IgD and IgE shows that the Asn-297 region on IgG is completely conserved in all three immunoglobulins, and may have a conserved role in folding, post-translational modification and function. Asn-265 in the Cε2 domain of IgE corresponds to Asn-297 of IgG.
EndoS is an endoglycosidase secreted by the human pathogen Streptococcus pyogenes. EndoS was identified as an enzyme which specifically hydrolyzes the Asn-297-linked glycan on IgG between the two core GlcNAc residues. In contrast to many related endoglycosidases that require or are enhanced by denaturation of the glycoprotein substrate, EndoS specifically hydrolyzes native IgG. No other substrate for EndoS has been reported.